List<T>.Exists(Predicate<T>) Método
Definição
public:
bool Exists(Predicate<T> ^ match);
public bool Exists (Predicate<T> match);
member this.Exists : Predicate<'T> -> bool
Public Function Exists (match As Predicate(Of T)) As Boolean
Parâmetros
- match
- Predicate<T>
O delegado Predicate<T> que define as condições dos elementos a serem pesquisados.The Predicate<T> delegate that defines the conditions of the elements to search for.
Retornos
true se o List<T> contiver um ou mais elementos que correspondem às condições definidas pelo predicado especificado; caso contrário, false.true if the List<T> contains one or more elements that match the conditions defined by the specified predicate; otherwise, false.
Exceções
match é null.match is null.
Exemplos
O exemplo a seguir demonstra Contains os Exists métodos e em um List<T> que contém um objeto comercial simples que implementa Equals .The following example demonstrates the Contains and Exists methods on a List<T> that contains a simple business object that implements Equals.
using System;
using System.Collections.Generic;
// Simple business object. A PartId is used to identify a part
// but the part name can change.
public class Part : IEquatable<Part>
{
public string PartName { get; set; }
public int PartId { get; set; }
public override string ToString()
{
return "ID: " + PartId + " Name: " + PartName;
}
public override bool Equals(object obj)
{
if (obj == null) return false;
Part objAsPart = obj as Part;
if (objAsPart == null) return false;
else return Equals(objAsPart);
}
public override int GetHashCode()
{
return PartId;
}
public bool Equals(Part other)
{
if (other == null) return false;
return (this.PartId.Equals(other.PartId));
}
// Should also override == and != operators.
}
public class Example
{
public static void Main()
{
// Create a list of parts.
List<Part> parts = new List<Part>();
// Add parts to the list.
parts.Add(new Part() { PartName = "crank arm", PartId = 1234 });
parts.Add(new Part() { PartName = "chain ring", PartId = 1334 });
parts.Add(new Part() { PartName = "regular seat", PartId = 1434 });
parts.Add(new Part() { PartName = "banana seat", PartId = 1444 });
parts.Add(new Part() { PartName = "cassette", PartId = 1534 });
parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ;
// Write out the parts in the list. This will call the overridden ToString method
// in the Part class.
Console.WriteLine();
foreach (Part aPart in parts)
{
Console.WriteLine(aPart);
}
// Check the list for part #1734. This calls the IEquatable.Equals method
// of the Part class, which checks the PartId for equality.
Console.WriteLine("\nContains: Part with Id=1734: {0}",
parts.Contains(new Part { PartId = 1734, PartName = "" }));
// Find items where name contains "seat".
Console.WriteLine("\nFind: Part where name contains \"seat\": {0}",
parts.Find(x => x.PartName.Contains("seat")));
// Check if an item with Id 1444 exists.
Console.WriteLine("\nExists: Part with Id=1444: {0}",
parts.Exists(x => x.PartId == 1444));
/*This code example produces the following output:
ID: 1234 Name: crank arm
ID: 1334 Name: chain ring
ID: 1434 Name: regular seat
ID: 1444 Name: banana seat
ID: 1534 Name: cassette
ID: 1634 Name: shift lever
Contains: Part with Id=1734: False
Find: Part where name contains "seat": ID: 1434 Name: regular seat
Exists: Part with Id=1444: True
*/
}
}
Imports System.Collections.Generic
' Simple business object. A PartId is used to identify a part
' but the part name can change.
Public Class Part
Implements IEquatable(Of Part)
Public Property PartName() As String
Get
Return m_PartName
End Get
Set(value As String)
m_PartName = Value
End Set
End Property
Private m_PartName As String
Public Property PartId() As Integer
Get
Return m_PartId
End Get
Set(value As Integer)
m_PartId = Value
End Set
End Property
Private m_PartId As Integer
Public Overrides Function ToString() As String
Return Convert.ToString("ID: " & PartId & " Name: ") & PartName
End Function
Public Overrides Function Equals(obj As Object) As Boolean
If obj Is Nothing Then
Return False
End If
Dim objAsPart As Part = TryCast(obj, Part)
If objAsPart Is Nothing Then
Return False
Else
Return Equals(objAsPart)
End If
End Function
Public Overrides Function GetHashCode() As Integer
Return PartId
End Function
Public Overloads Function Equals(other As Part) As Boolean _
Implements IEquatable(Of Part).Equals
If other Is Nothing Then
Return False
End If
Return (Me.PartId.Equals(other.PartId))
End Function
' Should also override == and != operators.
End Class
Public Class Example
Public Shared Sub Main()
' Create a list of parts.
Dim parts As New List(Of Part)()
' Add parts to the list.
parts.Add(New Part() With { _
.PartName = "crank arm", _
.PartId = 1234 _
})
parts.Add(New Part() With { _
.PartName = "chain ring", _
.PartId = 1334 _
})
parts.Add(New Part() With { _
.PartName = "regular seat", _
.PartId = 1434 _
})
parts.Add(New Part() With { _
.PartName = "banana seat", _
.PartId = 1444 _
})
parts.Add(New Part() With { _
.PartName = "cassette", _
.PartId = 1534 _
})
parts.Add(New Part() With { _
.PartName = "shift lever", _
.PartId = 1634 _
})
' Write out the parts in the list. This will call the overridden ToString method
' in the Part class.
Console.WriteLine()
For Each aPart As Part In parts
Console.WriteLine(aPart)
Next
' Check the list for part #1734. This calls the IEquatable.Equals method
' of the Part class, which checks the PartId for equality.
Console.WriteLine(vbLf & "Contains: Part with Id=1734: {0}",
parts.Contains(New Part() With { _
.PartId = 1734, _
.PartName = "" _
}))
' Find items where name contains "seat".
Console.WriteLine(vbLf & "Find: Part where name contains ""seat"": {0}",
parts.Find(Function(x) x.PartName.Contains("seat")))
' Check if an item with Id 1444 exists.
Console.WriteLine(vbLf & "Exists: Part with Id=1444: {0}",
parts.Exists(Function(x) x.PartId = 1444))
'This code example produces the following output:
'
' ID: 1234 Name: crank arm
' ID: 1334 Name: chain ring
' ID: 1434 Name: regular seat
' ID: 1444 Name: banana seat
' ID: 1534 Name: cassette
' ID: 1634 Name: shift lever
'
' Contains: Part with Id=1734: False
'
' Find: Part where name contains "seat": ID: 1434 Name: regular seat
'
' Exists: Part with Id=1444: True
'
End Sub
End Class
O exemplo a seguir demonstra o Exists método e vários outros métodos que usam o Predicate<T> delegado genérico.The following example demonstrates the Exists method and several other methods that use the Predicate<T> generic delegate.
Uma List<T> das cadeias de caracteres é criada, contendo 8 nomes de dinossauro, dois dos quais (às posições 1 e 5) terminam com "saurus".A List<T> of strings is created, containing 8 dinosaur names, two of which (at positions 1 and 5) end with "saurus". O exemplo também define um método de predicado de pesquisa chamado EndsWithSaurus , que aceita um parâmetro de cadeia de caracteres e retorna um valor booliano que indica se a cadeia de caracteres de entrada termina em "saurus".The example also defines a search predicate method named EndsWithSaurus, which accepts a string parameter and returns a Boolean value indicating whether the input string ends in "saurus".
Os Find FindLast métodos, e FindAll são usados para pesquisar a lista com o método de predicado de pesquisa e, em seguida, o RemoveAll método é usado para remover todas as entradas que terminam com "saurus".The Find, FindLast, and FindAll methods are used to search the list with the search predicate method, and then the RemoveAll method is used to remove all entries ending with "saurus".
Por fim, o Exists método é chamado.Finally, the Exists method is called. Ele atravessa a lista desde o início, passando cada elemento por vez para o EndsWithSaurus método.It traverses the list from the beginning, passing each element in turn to the EndsWithSaurus method. A pesquisa será interrompida e o método retornará true se o EndsWithSaurus método retornar true para qualquer elemento.The search stops and the method returns true if the EndsWithSaurus method returns true for any element. O Exists método retorna false porque todos esses elementos foram removidos.The Exists method returns false because all such elements have been removed.
Observação
No C# e no Visual Basic, não é necessário criar o representante Predicate<string> (Predicate(Of String) no Visual Basic) explicitamente.In C# and Visual Basic, it is not necessary to create the Predicate<string> delegate (Predicate(Of String) in Visual Basic) explicitly. Essas linguagens inferem o representante correto pelo contexto e criam-no automaticamente.These languages infer the correct delegate from context and create it automatically.
using namespace System;
using namespace System::Collections::Generic;
// Search predicate returns true if a string ends in "saurus".
bool EndsWithSaurus(String^ s)
{
return s->ToLower()->EndsWith("saurus");
};
void main()
{
List<String^>^ dinosaurs = gcnew List<String^>();
dinosaurs->Add("Compsognathus");
dinosaurs->Add("Amargasaurus");
dinosaurs->Add("Oviraptor");
dinosaurs->Add("Velociraptor");
dinosaurs->Add("Deinonychus");
dinosaurs->Add("Dilophosaurus");
dinosaurs->Add("Gallimimus");
dinosaurs->Add("Triceratops");
Console::WriteLine();
for each(String^ dinosaur in dinosaurs )
{
Console::WriteLine(dinosaur);
}
Console::WriteLine("\nTrueForAll(EndsWithSaurus): {0}",
dinosaurs->TrueForAll(gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nFind(EndsWithSaurus): {0}",
dinosaurs->Find(gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nFindLast(EndsWithSaurus): {0}",
dinosaurs->FindLast(gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nFindAll(EndsWithSaurus):");
List<String^>^ sublist =
dinosaurs->FindAll(gcnew Predicate<String^>(EndsWithSaurus));
for each(String^ dinosaur in sublist)
{
Console::WriteLine(dinosaur);
}
Console::WriteLine(
"\n{0} elements removed by RemoveAll(EndsWithSaurus).",
dinosaurs->RemoveAll(gcnew Predicate<String^>(EndsWithSaurus)));
Console::WriteLine("\nList now contains:");
for each(String^ dinosaur in dinosaurs)
{
Console::WriteLine(dinosaur);
}
Console::WriteLine("\nExists(EndsWithSaurus): {0}",
dinosaurs->Exists(gcnew Predicate<String^>(EndsWithSaurus)));
}
/* This code example produces the following output:
Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops
TrueForAll(EndsWithSaurus): False
Find(EndsWithSaurus): Amargasaurus
FindLast(EndsWithSaurus): Dilophosaurus
FindAll(EndsWithSaurus):
Amargasaurus
Dilophosaurus
2 elements removed by RemoveAll(EndsWithSaurus).
List now contains:
Compsognathus
Oviraptor
Velociraptor
Deinonychus
Gallimimus
Triceratops
Exists(EndsWithSaurus): False
*/
using System;
using System.Collections.Generic;
public class Example
{
public static void Main()
{
List<string> dinosaurs = new List<string>();
dinosaurs.Add("Compsognathus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("Oviraptor");
dinosaurs.Add("Velociraptor");
dinosaurs.Add("Deinonychus");
dinosaurs.Add("Dilophosaurus");
dinosaurs.Add("Gallimimus");
dinosaurs.Add("Triceratops");
Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
Console.WriteLine(dinosaur);
}
Console.WriteLine("\nTrueForAll(EndsWithSaurus): {0}",
dinosaurs.TrueForAll(EndsWithSaurus));
Console.WriteLine("\nFind(EndsWithSaurus): {0}",
dinosaurs.Find(EndsWithSaurus));
Console.WriteLine("\nFindLast(EndsWithSaurus): {0}",
dinosaurs.FindLast(EndsWithSaurus));
Console.WriteLine("\nFindAll(EndsWithSaurus):");
List<string> sublist = dinosaurs.FindAll(EndsWithSaurus);
foreach(string dinosaur in sublist)
{
Console.WriteLine(dinosaur);
}
Console.WriteLine(
"\n{0} elements removed by RemoveAll(EndsWithSaurus).",
dinosaurs.RemoveAll(EndsWithSaurus));
Console.WriteLine("\nList now contains:");
foreach(string dinosaur in dinosaurs)
{
Console.WriteLine(dinosaur);
}
Console.WriteLine("\nExists(EndsWithSaurus): {0}",
dinosaurs.Exists(EndsWithSaurus));
}
// Search predicate returns true if a string ends in "saurus".
private static bool EndsWithSaurus(String s)
{
return s.ToLower().EndsWith("saurus");
}
}
/* This code example produces the following output:
Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops
TrueForAll(EndsWithSaurus): False
Find(EndsWithSaurus): Amargasaurus
FindLast(EndsWithSaurus): Dilophosaurus
FindAll(EndsWithSaurus):
Amargasaurus
Dilophosaurus
2 elements removed by RemoveAll(EndsWithSaurus).
List now contains:
Compsognathus
Oviraptor
Velociraptor
Deinonychus
Gallimimus
Triceratops
Exists(EndsWithSaurus): False
*/
Imports System.Collections.Generic
Public Class Example
Public Shared Sub Main()
Dim dinosaurs As New List(Of String)
dinosaurs.Add("Compsognathus")
dinosaurs.Add("Amargasaurus")
dinosaurs.Add("Oviraptor")
dinosaurs.Add("Velociraptor")
dinosaurs.Add("Deinonychus")
dinosaurs.Add("Dilophosaurus")
dinosaurs.Add("Gallimimus")
dinosaurs.Add("Triceratops")
Console.WriteLine()
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & _
"TrueForAll(AddressOf EndsWithSaurus: {0}", _
dinosaurs.TrueForAll(AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"Find(AddressOf EndsWithSaurus): {0}", _
dinosaurs.Find(AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"FindLast(AddressOf EndsWithSaurus): {0}", _
dinosaurs.FindLast(AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & _
"FindAll(AddressOf EndsWithSaurus):")
Dim sublist As List(Of String) = _
dinosaurs.FindAll(AddressOf EndsWithSaurus)
For Each dinosaur As String In sublist
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & _
"{0} elements removed by RemoveAll(AddressOf EndsWithSaurus).", _
dinosaurs.RemoveAll(AddressOf EndsWithSaurus))
Console.WriteLine(vbLf & "List now contains:")
For Each dinosaur As String In dinosaurs
Console.WriteLine(dinosaur)
Next
Console.WriteLine(vbLf & _
"Exists(AddressOf EndsWithSaurus): {0}", _
dinosaurs.Exists(AddressOf EndsWithSaurus))
End Sub
' Search predicate returns true if a string ends in "saurus".
Private Shared Function EndsWithSaurus(ByVal s As String) _
As Boolean
Return s.ToLower().EndsWith("saurus")
End Function
End Class
' This code example produces the following output:
'
'Compsognathus
'Amargasaurus
'Oviraptor
'Velociraptor
'Deinonychus
'Dilophosaurus
'Gallimimus
'Triceratops
'
'TrueForAll(AddressOf EndsWithSaurus: False
'
'Find(AddressOf EndsWithSaurus): Amargasaurus
'
'FindLast(AddressOf EndsWithSaurus): Dilophosaurus
'
'FindAll(AddressOf EndsWithSaurus):
'Amargasaurus
'Dilophosaurus
'
'2 elements removed by RemoveAll(AddressOf EndsWithSaurus).
'
'List now contains:
'Compsognathus
'Oviraptor
'Velociraptor
'Deinonychus
'Gallimimus
'Triceratops
'
'Exists(AddressOf EndsWithSaurus): False
Comentários
O Predicate<T> é um representante para um método que retorna true caso o objeto passado para ele corresponda às condições definidas no representante.The Predicate<T> is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. Os elementos da atual List<T> são passados individualmente para o Predicate<T> delegado e o processamento é interrompido quando uma correspondência é encontrada.The elements of the current List<T> are individually passed to the Predicate<T> delegate, and processing is stopped when a match is found.
Esse método executa uma pesquisa linear; Portanto, esse método é uma operação O (n), onde n é Count .This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.